Monitoring the temperature over time for a temperature-sensitive, perishable element is important to accurately determine its lifetime and decide when the perishable element should no longer be used and therefore disposed of. Perishable elements may include certain foods, drugs, biological specimens and materials that age as a function of time and temperature. Being able to accurately determine the thermal history of a perishable element over time is necessary in the food industry to prevent people from getting sick or dieing from expired food. The effectiveness of some drugs and materials also decreases over time and their continued use beyond a suggested lifetime may lead to detrimental effects contraindicated to their intended use. On the other hand, disposing of a perishable element before necessary may lead to throwing away inventory that may have significant monetary value. It is therefore important to have an accurate thermal history of what the perishable element actual sees over time to both minimize risk and save money.
The problem with accurately monitoring the temperature history that a perishable element sees, say in a cold refrigerator or freezer, is that the air temperature within the refrigeration unit changes dramatically each time the door is opened. The air temperature may change by tens to hundreds of degrees with each opening depending on the difference between the inside and outside temperature. The perishable element, however, because of its thermal mass and insulating properties may have its temperature change by only a few degrees during each exposure. Recording just the temperature changes of the air within the refrigerator over time will therefore over estimates the time-temperature exposure for the perishable element. This may lead to the perishable element being disposed before it needs to be and will result in throwing perfectly good product away.
To better characterize the temperature of a perishable device over time, a variety of perishable element simulating and sensing devices have been developed. One example is a bottle filled with liquid that has a thermometer, such as H-B Instrument's DURACPLUS® bottle thermometers. These thermometers insulate the sensing part of the thermometer with a liquid such as glycol. These products require visual reading that are subject to user error and may be easily broken if dropped. Also, any liquids that may leak from the bottle may contaminate nearby perishable elements. Another example is illustrated in U.S. Pat. No. 3,964,313 to Jones where a food can of the same composition and physical dimensions as the container of food usually stored in the container is filled with a porous open cell material distributed with a liquid to simulate the food. These types of simulators again use liquids, require complex fabrication techniques and are cumbersome to use. It is therefore apparent that the thermal monitoring industry would benefit from more accurate, durable and simpler thermal simulators.
Another issue with monitoring the temperature over time for a temperature-sensitive, perishable element is whether the monitoring sensing device itself is calibrated and reading properly. Sensing devices need to be checked periodically over time to insure that during the period of operation, they were actually taking proper measurements. Depending on the application and industry, calibration monitoring may need to take place every couple weeks to every several months. Having to remove, calibrate and reinstall monitoring sensing devices on a regular basis can become a very time consuming and costly endeavor. In situ calibration is best, but current state of the art solid thermal simulator sensing devices do not have that capability.
The present invention provides for a thermal device that simulates a perishable element and allows the user to more accurately monitor the perishable element's temperature over time. The thermal device is durable and safe. The thermal device is also structured to facilitate in situ calibration that requires minimal time and effort.